Embodiments of the invention relate to a water-based primer coating for use on paper and polymeric substrates, and more particularly, to a polyurethane-based primer coating for enhancing the adhesion of liquid toner to substrates which are designed to be printed using an electrophotographic printing device.
In recent years, the use of on-demand digital printing machines utilizing liquid electrophotographic systems has become widespread. In such systems, liquid toner images are formed on a photosensitive drum and then electrically transferred onto an intermediate transfer blanket or belt for printing on a paper or polymeric film sheet or web. Printers using such toners or inks are commercially available from Hewlett-Packard Company under the trade name HP Indigo.
However, it is well known in the art that liquid toners to do not transfer well and/or adhere well to films or paper substrates unless those films or paper substrates have been treated with a coating or primer to enhance their adhesion to a substrate.
A number of primers have been developed for use on substrates such as polymeric films which render the surface of the films more receptive to liquid toners. Typical primers currently in use have been developed based on ethylene-acrylic acid copolymers. As described in commonly-assigned U.S. Pat. No. 7,470,736, one such primer coating includes a copolymer of ethylene and acrylic or methacrylic acid and a compatible adhesion enhancer which is used to enhance the adhesion of liquid toner to a variety of polymeric substrates.
However, the development of newer, high speed digital presses has presented new challenges to the use of such primers for liquid toner transfer and adhesion to the substrate. For example, the more recently developed HP Indigo 6000 series of digital presses offer higher speed printing at about twice the speed of conventional printing presses. These higher speeds require shorter toner to substrate transfer times, which in turn require lower activation temperatures for the primer to effectively enhance toner transfer and adhesion to the printed substrate. As a result, substrates coated with existing primers may provide poor toner transfer and adhesion when run through the newest generation of digital printing presses.
Accordingly, there is a need in the art for a primer for use with high speed digital printing presses which provides good liquid toner transfer and adhesion to a variety of substrates.